Most energy losses from fluid power systems occur as a result of decreases in fluid pressure that do not accomplish useful work. Such pressure drops dissipate energy expended to pressurize the fluid in the form of heat.
For example, fixed displacement pumps are generally sized to meet a maximum system demand for a rate of fluid flow at a given pressure even though the maximum demand occurs only rarely. Any portion of the pressurized flow that is not required to meet a particular demand is exhausted to a return side of the fluid system by a pressure control relief valve, whose primary function is to limit system pressure. A product of the volume of exhausted fluid and its drop in pressure equals the amount of energy that is lost through the relief valve.
Similar energy losses are associated with fixed displacement motors that convert fluid power in the form of flow rate and pressure into mechanical power in the form of rotational speed and torque. Flow control valves, whose primary function is to control flow rate, are often used as throttle valves to regulate the rotational speed of fixed displacement motors. However, if a pressure drop across the motor (corresponding to a particular output torque of the motor) is less than the system pressure, then a second pressure drop equal to the difference pressure occurs across the throttle valve. The amount of energy lost is equal to a product of the volume of fluid passing through the throttle valve and the drop in pressure across the valve.
Many different approaches have been taken to minimize energy losses associated with fixed displacement pumps and motors. One approach to minimizing such energy losses is to control the rotational speed of a prime mover driving the fixed displacement pump as a function of system pressure. Electrical energy used to power an electric motor as the prime mover is saved by reducing the rotational speed of the electric motor in response to an increase in system pressure. However, this approach has been limited mainly to small electric motors as prime movers, because large electric motors are more difficult to operate efficiently at varying speeds and react much more slowly to desired changes in speed.
Another approach to energy savings with fixed displacement pumps and motors has been to replace single fixed displacement pumps and motors with respective gangs of smaller fixed displacement pumps and motors having the same total displacement. For example, U.S. Pat. No. 4,199,943 to Hunt discloses a fluid pumping system in which a gang of fixed displacement pumps are driven in unison by a prime mover. The pumps draw fluid from a common reservoir and output the fluid through respective diverter valves to either a motor supply line or the reservoir. The diverter valves are biased into positions that connect each of the pumps to the motor supply line. However, the diverter valves are controlled to successively divert the pump output flow to the reservoir in response to pressure increases in the motor supply line.
U.S. Pat. No. 4,245,964 to Rannenberg saves energy in a similar manner by mechanically connecting a pair of fixed displacement pumps to a common prime mover and deactivating one of the pumps in response to an increase in system pressure. However, instead of exhausting output flow from the deactivated pump to a reservoir, the output flow is recirculated through the deactivated pump so that there is no pressure differential across the pump.
The pumping systems of both Hunt and Rannenberg can be used to save energy by better matching power demands on the prime mover with power expended by the fluid power system to accomplish useful work. More particularly, both systems save energy by better matching the effective displacements of their pump systems to demands for fluid flow at given pressures. However, the effective displacements of their pump systems can be varied only by large increments, and power is wasted when the demand for fluid flow cannot be exactly matched by the available increments of displacement.
Fluid transmissions have also been arranged in a similar manner to conserve power. For example, U.S. Pat. No. 986,780 to Sundh and U.S. Pat. Nos. 2,370,526 and 2,374,588 to Doran disclose fluid transmissions in which respective gangs of fixed displacement motors are connected to common output shafts. Each of the transmissions includes a fixed displacement pump that provides a source of fluid flow to the gang of motors and one or more valves that provide for successively connecting the motors to the fluid flow. Output torque of the transmission increases and output rotational speed decreases as more of the motors are connected.
Although such fluid transmissions are operable over a wide range of output speeds while delivering most of the available fluid power to a load, the actual output speed must be controlled by other means. However, if the output speed is controlled (e.g., by limiting the flow rate through the transmission with a throttle valve), then less than all of the available fluid power may be delivered by the transmission (e.g., the pressure drop across the transmission is less than the system pressure) and the unused power may be wasted. In other words, energy losses from transmissions including respective gangs of fixed displacement motors are similar to losses from individual fixed displacement motors when less than all of the available fluid power is delivered by the transmissions or individual motors.
The attempts to reduce energy losses from fluid power systems also include replacing fixed displacement pumps and motors with variable displacement pumps and motors. However, with respect to fixed displacement pumps and motors, variable displacement pumps and motors are larger, more expensive, less reliable, and more sensitive to fluid contaminants; require more maintenance; and are subject to catastrophic failure that can do far-ranging damage to the fluid system in which they are used. Fixed displacement pumps and motors, such as gear pumps and motors, wear in a very gradual and predictable manner that is easily monitored by their performance in conducting routine maintenance or replacement.
Variable displacement pumps and motors have also been used to supplement operations of fixed displacement pumps and motors to reduce energy losses. For example, U.S. Pat. No. 3,038,312 to Marsh discloses a hydrostatic transmission in which a device operable as either a variable displacement pump or a variable displacement motor is mechanically coupled to a fixed displacement pump driven by a prime mover. As a pump, the variable displacement device supplements the operation of the fixed displacement pump by delivering variable amounts of additional flow to a fixed displacement motor. However, when the motor's demand for fluid flow is less than the output flow of the fixed displacement pump, the variable displacement device is operated as a motor that receives variable amounts of the flow from the fixed displacement pump and contributes to the pump's rotation.
The variable displacement device of Marsh operating as a motor replaces a conventional throttle valve interrupting a motor by-pass line for controlling the rotational speed of a fixed displacement motor in a hydrostatic transmission. That is, the variable displacement device (motor) of Marsh provides a conventional throttling function, but also converts a pressure drop across the variable displacement motor into useful work by turning the fixed displacement pump. Energy reclaimed from the pressure drop is used to reduce input power demands of the prime mover. The use of a pressure drop, which would otherwise result in a loss of energy from a fluid system, to accomplish useful work is referred to as "regeneration".
A similar type of regeneration is achieved in a hydrostatic transmission disclosed in U.S. Pat. No. 3,203,185 to Williams by replacing a counterbalance pressure control valve in a return line of the transmission with a variable displacement pump. Ordinarily, such a counterbalance valve would be used to maintain a predetermined backpressure in the return line to prevent variations in a load from overdriving an output shaft of the transmission. However, a pressure drop across the counterbalance valve dissipates a large amount of energy required to pressurize the fluid as heat.
In place of the counterbalance valve, Williams connects the variable displacement pump between an output port of a fixed displacement motor that drives the transmission output shaft and an input port of a fixed displacement pump that provides a source of fluid flow. The variable displacement pump is driven by the fixed displacement motor through a common mechanical connection with the output shaft, and the pump's displacement is varied to maintain a predetermined pressure in the return line to the fixed displacement pump. The pressurized fluid in the return line provides the fixed displacement pump with a supply of fluid that reduces the amount of energy required by the fixed displacement pump to raise the pressure of the fluid to system pressure. In other words, the differential pressure across the fixed displacement pump is reduced by the backpressure.
U.S Pat. No. 2,549,989 to Simonds discloses a multiple motor fluid transmission system that also achieves regenerative effects by coupling variable displacement devices to each of three fixed displacement motors. A primary variable displacement pump driven by a prime mover provides a supply of fluid at a constant pressure to each of the fixed displacement motors and to each of the variable displacement devices that are respectively coupled to the motors.
When a load exceeds the torque capacity of one of the motors, the associated variable displacement device is arranged to function as a motor for supplementing the torque output of the fixed displacement motor. However, when the torque capacity of one of the motors exceeds the requirements of a load, the associated variable displacement device is arranged to function as a pump for using the excess capacity of the fixed displacement motor to add to the supply of pressurized fluid. In other words, if the pressure drop across one of the fixed displacement motors is less than the system pressure, the remaining pressure is used to drive a variable displacement pump that returns part of the flow from the motor to system pressure. The increased flow of pressurized fluid reduces the demand for fluid flow from the primary variable displacement pump supplying the system.
The regenerative fluid systems of Marsh, Williams, and Simonds save energy by using variable displacement devices in place of throttle valves or relief valves for converting pressure drops in their systems into useful work. Although possibly smaller than stand-alone variable displacement pumps and motors, the known variable displacement devices that are mechanically connected to fixed displacement pumps or motors for regenerative purposes have the same disadvantages as stand-alone variable displacement pumps and motors. Accordingly, the known regenerative systems have limited practical applications.